The present invention relates to a data communication apparatus and a data communication method for transmitting/receiving a binary image.
In a conventional facsimile apparatus, when data transmission/reception is to be performed, a transmission side first scans an original using a photoelectric transducer such as a CCD, compresses an output signal from the photoelectric transducer into black-and-white binary data by binarization processing, and then sends the binary data to a reception side.
As the binarization processing, an operator selects an optimal binarization processing method upon operation of an external key in accordance with a type of original (e.g., a character original, a photograph original, a character-photograph mixed original, or the like).
For example, for a character original, binarization processing is attained by fixed threshold value processing, and for a photograph original or a photograph-character mixed original, binarization processing is attained by an error diffusion method or a dither method.
Binary data is encoded by a known MR (Modified Read) method, or an MH (Modified Huffman) method, and the encoded data is transmitted to a communication apparatus of a called party.
Upon selection of an encoding method such as the MR or MH method, in, for example, a G3 standard facsimile apparatus, a calling parity detects an encoding method of a called party on the basis of a DIS signal from the called party, and then determines a most efficient encoding method for both the calling and called parties from communication possible encoding methods, i.e., preferentially selects the MR method in the above-mentioned case as an encoding method.
Table 1 summarizes the relationship upon selection of encoding methods.
The relationship shown in Table 1 is established since a compression ratio of encoding by the MR method is higher than that by the MH method when general document information is subjected to communications.
As shown in Table 1, conventionally, of encoding and decoding methods of transmission and reception sides, a method which can minimize an information volume upon communication of general document information is selected, and the MR encoding method is preferentially selected.
However, image data which is binarized by a pseudo halftone processing method (a density preservation type binarization processing method such as an error diffusion method, a dither processing method, or the like) which is used to binarize a photograph original, of the binarization processing methods is preferably encoded by the MH encoding method rather than the MR encoding method to perform encoding with a higher compression ratio.
Therefore, in a conventional encoding method in which the MR encoding method is most preferentially selected, data is encoded by the MR encoding method which may impair a compression ratio to perform data transmission despite that a higher compression ratio may be attained by the MH encoding method depending on a binarization processing method.
This leads to an increase in communication time and communication charge.